The present invention relates to implantable medication infusion devices and, more particularly to such devices which are arranged to provide a continuous unprogrammed flow of medication into the body. Still more particularly, the invention relates to devices of the aforementioned type which are particularly adapted to be implanted in farm animals such as cows or hogs and to a bolus generator used with such devices for periodically dispensing a bolus of medication.
An implantable medication infusion device of the continuous flow type is shown in my U.S. Pat. No. 4,626,244 issued Dec. 2, 1986. A relatively constant pressure is exerted on a diaphragm forming at least one wall of a reservoir containing medication. The pressure exerted on the diaphragm is above body pressure so that medication is forced out of the reservoir through a silicon substrate flow restriction device and a catheter to the infusion site within the body. That device is intended primarily for implantation in human bodies. Such devices are usually made particularly small and thin. Moreover, devices of the aforementioned type commonly use a two phase fluid (both gas and liquid coexisting at the fluid vapor pressure) such as Freon 11 produced by E. I. DuPont Co. or FC-87 or FC-88 Perfluorocarbon, produced by the 3M Company as the propellant for generating pressure on the diaphragm. These two phase fluids require the use of metal housing and reservoir parts for compatibility with the fluid. These and other factors to be considered in devices of the aforementioned type suitable for use in humans result in relatively high costs for the device.
It is known that infusion of growth hormones produced by recombinant DNA and possibly other drugs provide benefit in the animals such as an increased quantity of milk production in cows, larger growth of the animals to increase weight and edible meat, a reduced fat content of meat and an improved feed efficiency for the animals, among other things. Presently, such drugs are injected into the animals as often as once a day by syringe/needle bolus injection. Such procedures are time consuming and costly.
This invention provides an implantable device which can deliver a drug continuously or alternatively in a bolus fashion more than once a day for a prolonged period of time, requiring refill by percutaneous syringe/needle injection only once every several months. The device is made inexpensively of non-metallic materials and consists mainly of molded plastic and rubber parts. It is generally essential that such a device implanted in farm animals be inexpensive so that its cost does not offset the cost reduction effected by drug infusion. The device of this invention uses pressurized air or inert gas as the propellant acting upon the medication reservoir for forcing the drug out of the device to the infusion site. A silicon substrate flow restrictor is utilized to reduce the size and cost of this element. A fixed barrier is provided within the device to constrain an elastomer bladder forming a part of the medication reservoir against stretching, so as to eliminate added pressure on the drug which would be caused by contraction of the bladder if it were stretched. In certain drug applications, an efficacy improvement can be obtained when the drug is dispensed in repetitive boli rather than as a continuous flow. Previous devices have utilized electronically programmable electromagnetic pumps for generating periodic dosages of the drug. This invention provides a bolus generator having no moving parts which may be attached to the output end of a catheter for the device and located at the infusion site to dispense repeating boli of the drug.
The invention and its advantages will be more readily understood when reading the following description and claims in conjunction with the accompanying drawings.